1. Field of the Invention
This invention relates to an output control system for a series hybrid vehicle, more particularly to an output control system for a series hybrid vehicle that controls the output (power) of a generator-motor so as to achieve the input required by an electric motor for driving wheels at the engine speed providing optimum fuel efficiency in real time.
2. Description of the Related Art
Hybrid vehicles that combine a generator-motor and an internal combustion engine are of two general types: series hybrid vehicles whose wheels are driven solely by an electric motor (generator-motor) and parallel hybrid vehicles whose wheels are driven by either or both an electric motor (generator-motor) or and an internal combustion engine.
A series hybrid vehicle is generally equipped with an internal combustion engine, a first generator-motor connected with the engine, a battery (electrical energy storage means) connected with and charged by the first generator-motor, and an electric motor (second generator-motor) for driving the wheels that is powered by the output of either the first generator-motor or the battery. Various techniques have been proposed for controlling the output of the first generator-motor of such a series hybrid vehicle.
Japanese Laid-Open Patent Application No. Hei 5(1993)-146,008, for example, teaches a system for achieving power distribution without causing mechanical shock that responds to the input required by the wheel drive motor by selecting one mode from among "battery only," "generator-motor only " and "battery plus generator-motor," determines the generator-motor output voltage needed to effect the required input in the selected mode, and, while monitoring the generator-motor output voltage, battery output voltage, engine speed or generator-motor output voltage, controls the field current of the generator-motor to obtain the determined generator-motor output.
More specifically, this conventional system attempts to improve fuel efficiency by operating the internal combustion engine in the range between 100% and 60% of maximum output and effecting operation outside this operating range in combination with the battery or by the battery alone.
However, the way these conventional technologies deal with the wheel drive motor being operated at maximum output is to add the battery output to the generator-motor output. Therefore, at a time when the wheel drive motor is operating at maximum output, if an attempt should be made to secure all power required by the wheel drive motor solely from the generator-motor without relying on the battery, the internal combustion engine would have to be operated over a relatively broad output range.
From the viewpoint of fuel efficiency, an internal combustion engine should preferably be operated at the BSFC (Brake Specific mean Fuel Consumption) point, i.e., the point where fuel consumption is minimum. The conventional technologies leave room for fuel efficiency improvement, particularly when operation over a relatively broad range is attempted as discussed above.
Moreover, when the on-board battery of the vehicle is relatively small, and also when the required input increases momentarily in response to the operating state, supply of the required input must be controlled in real time.